[lammps-users] Bond quartic question

From:

Carsten Svaneborg <public@...4232...>

Date:

Thu, 20 Jul 2017 11:02:28 +0200

Dear List,
I'm somewhat puzzled about the implementation of bond quartic.
The documentation:

> If a bond length ever becomes > Rc, LAMMPS “breaks” the bond, which
means two things.
> First, the bond potential is turned off by setting its type to 0, and
is no longer computed.
> Second, a pairwise interaction between the two atoms is turned on,
since they are no
> longer bonded. LAMMPS does the second task via a computational
sleight-of-hand.
> It subtracts the pairwise interaction as part of the bond
computation. When the bond breaks,
> the subtraction stops. For this to work, the pairwise interaction
must always be computed by
> the /pair_style/ <http://lammps.sandia.gov/doc/pair_style.html>
command, whether the bond is broken or not. This means that
/special_bonds/ <http://lammps.sandia.gov/doc/special_bonds.html>

> must be set to 1,1,1, as indicated as a restriction below.

The code does exactly this. But for two bonded beads, why is LAMMPS
calculating a WCA force contribution in the bond, then subtracting a WCA
pair force again via force->pair->single(..) in the bond, and finally
adding it again via the pair calculation?

With special bonds 111, it would be much simpler just to define the bond
potential WITHOUT a WCA contribution, and just switch this off the bond
when it is broken, and hence always calculate the WCA term as part of
the pair interaction independently of whether beads are bonded or not.

The only difference, I can see, is that the partitioning of energies
between Epair and Ebond would differ when bonds are broken, but this is
a philosophical question. But it would make more sense with Ebond =0 in
the limit where all bonds are broken.